1. A plasma membrane: This outer boundary encloses the cell, regulating what enters and exits. It's essentially a selectively permeable barrier, maintaining the cell's internal environment.

2. DNA: This genetic material carries the instructions for building and maintaining the cell. It's the blueprint for life, dictating what proteins the cell produces and how it functions.

3. Ribosomes: These tiny structures are the protein factories of the cell. They use the instructions encoded in DNA to create proteins, which are essential for all cellular functions.

These three characteristics are the core components that define what makes something a cell.